This invention relates to a method of making oxydiphthalic anhydride (ODPA) from oxydiphthalic acid (ODTA). In particular, it relates to the use of both heat and acetic anhydride to obtain a virtually complete reaction of ODTA to ODPA in a short time without degradation of the ODPA.
ODPA is used to make polyimides and other chemical products. In certain applications, such as in the electronics industry, it is necessary for these products to have a low level of impurities, especially of ionic impurities. The purification of ODPA can be accomplished by converting the ODPA to ODTA, then reconverting the ODTA back to ODPA.
The conversion of ODPA to ODTA is relatively simple and it occurs readily with the addition of water. ODTA is almost insoluble in the water, but most impurities, especially ionic impurities, are soluble, and in this way they can be separated from ODTA. It is the conversion of the ODTA back to ODPA that presents the problems.
The conversion of ODTA to ODPA can be accomplished by heating ODTA to a temperature below its melting point (227.degree. C.). However, conversions below the melting point cannot be completed in a reasonable time. If the conversion is incomplete, a small amount, less than 1 wt %, of a compound known as oxydiphthalic diacid (ODDA) is produced. In ODDA one of the diacid groups of ODTA is converted to anhydride while the other diacid group remains a diacid. In the polymerization of the ODPA with a comonomer such as a diamine (to make a polyimide), the ODDA acts as a chain terminator, resulting in low molecular weight polymer of inferior properties. Thus, it is necessary to reduce the ODDA content of the ODPA to below the detectable limit of about 0.05 wt % in order to produce high molecular weight polymers.
This can be accomplished in one of four ways. First, the ODTA can be heated at very close to its melting point for a long time. However, this requires careful temperature control in order to avoid going over the melting point and the length of time required is so long that it may be impractical for most industrial purposes. Second, the ODTA can be heated above its melting point. However, this can cause degradation of the ODPA which can also result in lower molecular weight polymers. The third method is to react the ODTA with a large excess of a dehydrating agent such as acetic anhydride (see, for example, U.S. Pat. No. 4,808,731). Acetic anhydride dehydrates ODTA chemically as it reacts with ODTA to form ODPA and acetic acid. The problem with this method is that 8 moles or more of excess acetic anhydride must be employed. The unreacted acetic anhydride must then be recovered, disposed of, or further processed, which adds additional costs to the process. If less than the stoichiometric amount (i.e., two moles/mole of ODTA) of acetic anhydride is used, of course, only a portion of the ODTA will be converted to ODPA.
Another problem with the use of acetic anhydride is its harmful effects. Even when exhaustively dried, the undesirable odor of acetic anhydride lingers with ODPA that is cyclized in acetic anhydride. Furthermore, the odor detection limit of acetic anhydride in air is reported to be 0.14 to 0.36 ppm, and since the Occupational Safety and Health Administration (OSHA) permissible exposure limit is 5 ppm, potential health hazards exist because there is only a small concentration window in which the odor can be detected at a level considered safe.
A fourth method of converting ODTA to ODPA is to recrystallize ODTA in a high boiling solvent such as 1,2-dichlorobenzene. ODTA would be refluxed in the solvent. As cyclization occurs, water is taken off overhead. When the solution clears, the cyclization is complete and the solution can be cooled to crystallize ODPA which is then recovered by filtration. The ODPA filter cake must then be dried to remove the solvent. The problem is that residual solvent remains with the ODPA, particularly when the solvent is dichlorobenzene (DCB), and the process can take 8 hours or more.